Sentry

崗哨.
(Sentry.)
(picture source：photo by this team)

　　The final line of defense of the prison. When staff switches shifts, the doors must be opened from the outside; in other words, without someone to open the sentry locks from the outside, one cannot exit. When changing guards, someone must open the door for you, and you have to lock the door for them.

崗哨
(Sentry)
(picture source：photo by this team)